Piston to valve clearance issues.
- Thread starter dmh5015
- Start date
Measure your piston to valve clearance to be absolutely certain. You need to stay at or above .110" for your intake valve and .080" for the exhaust.
You will typically want more clearance on the exhaust valve due to heat expansion.
General rule of thumb is .080" for the intake valve and .100" for the exhaust valve.
As the powerband goes up, the clearance needs to be increased.
Lift has very little to do with piston to valve clearance.
I would look into individual valve timing events (IO, IC, EO, EC) and the amount of time the valves are open (duration).
Early intake opening and late exhaust valve closings are a big contributer to piston to valve contact. This is known as the period of overlap, when both valves are open at the same time.
It is not based off peak gross lift.
Now, I would be careful on getting a thicker headgasket, because that will hurt the quench area a bit.
With your heads being shaved .040", you need to check piston to valve clearance yourself
It just makes it that much more crucial.
General rule of thumb is .080" for the intake valve and .100" for the exhaust valve.
As the powerband goes up, the clearance needs to be increased.
Lift has very little to do with piston to valve clearance.
I would look into individual valve timing events (IO, IC, EO, EC) and the amount of time the valves are open (duration).
Early intake opening and late exhaust valve closings are a big contributer to piston to valve contact. This is known as the period of overlap, when both valves are open at the same time.
It is not based off peak gross lift.
Now, I would be careful on getting a thicker headgasket, because that will hurt the quench area a bit.
With your heads being shaved .040", you need to check piston to valve clearance yourself
It just makes it that much more crucial.
5spd GT said:
Did I have my intake/exhaust figures backwards? It's possible, I was going by memory...
Yeah, you just flip-flopped them, that's all
.080" for the intake and .100" for the exhaust.
Some say, .100" for the intake and .125" for the exhaust.
Makes sense, now that I think about it. You want more clearance on the exhaust valve because the piston is coming up towards it, so that leaves the most room for interference.
It is more for heat expansion from my understanding, because of the hot compressed gasses that are pushed around and out the exhaust valve.
The valve timing can vary. Some camshaft's valve timing is going to have the exhaust close before, at, or after top dead center (TDC).
Just as well, some intake valves begin to open before, at, or after top dead center.
So it really all depends on the camshaft.
Intake valves, as you know, have a larger radius as well which is usually how piston to valve contact is made. Especially on small valve relief pistons, like the stockers. The valve is usually a good ways outside of the valve relief.
Steep ramp rates that open up the valves quickly are also a big reason for piston to valve clearance.
But, along the lines of what you were stating, late exhaust valve closing, or early intake valve openings (BTDC) can cause issues.
Then you get into the valve relief size and angle and the approach angle of the valves.
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